In recent years, it has become common to transmit not only speech data but also a large amount of data such as still image data and moving image data along with the adoption of multimedia information in cellular mobile communication systems. In order to achieve transmission of a large amount of data, studies have been actively carried out on technologies that achieve a high data rate using a high-frequency radio band.
When a high-frequency radio band is utilized, however, attenuation increases as the transmission distance increases, although a higher data rate can be expected with a short distance. Accordingly, the coverage area of a radio communication base station apparatus (hereinafter, abbreviated as “base station” or “evolved Node B (eNB)”) decreases when a mobile communication system using a high-frequency radio band is actually put into operation. Thus, more base stations need to be installed in this case. The installation of base stations involves reasonable costs, however. For this reason, there has been a high demand for a technique that provides a communication service using a high-frequency radio band, while limiting an increase in the number of base stations.
In order to meet such a demand, studies have been carried out on a relay transmission technique in which a radio communication relay station apparatus (hereinafter, abbreviated as “relay station” or “relay node (RN)”) is installed between a base station and a radio communication mobile station apparatus (hereinafter, abbreviated as “mobile station” or “user equipment (UE)”) to perform communication between the base station and mobile station via the relay station for the purpose of increasing the coverage area of each base station. The use of relay technique allows a mobile station not capable of directly communicating with a base station to communicate with the base station via a relay station. For example, relay station 20 is installed between base station 10 and mobile station 30, and base station 10 and mobile station 30 communicate with each other via relay station 20 in a radio relay system illustrated in FIGS. 1A and 1B.
(TD Relay)
The Long Term Evolution Advanced (LTE-A) system for which the introduction of relay technique has been studied is required to maintain compatibility with Long Term Evolution (LTE) in terms of a smooth transition from and coexistence with LTE. For this reason, mutual compatibility with LTE is required for the relay technique as well. In the LTE-A system, studies have been carried out on configuring MBMS single frequency network (MBSFN) subframes for downlink (hereinafter, abbreviated as “DL”) transmission from a base station to a relay station in order to achieve mutual compatibility with LTE.
The base station and mobile station communicate with each other via the relay station herein using time-division relay (i.e., TD relay). FIGS. 1A and 1B are diagrams provided for describing the TD relay. FIG. 1A is a conceptual diagram for describing downlink TD relay, and FIG. 1B is a conceptual diagram for describing uplink TD relay. In the TD relay (also referred to as “half duplex relay” or “Type 1 relay”), transmission from a base station to a relay station and transmission from the relay station to a mobile station are divided in time.
As illustrated in FIG. 1B, transmission is performed from mobile station 30 to relay station 20 on the access link in subframe #2 while communication from relay station 20 to base station 10 is performed on the backhaul link in subframe #3 in uplink. In subframe #4, transmission is performed from mobile station 30 to relay station 20 again.
Likewise, as illustrated in FIG. 1A, transmission is performed from relay station 20 to mobile station 30 on the access link in subframe #2 while communication is performed from base station 10 to relay station 20 on the backhaul link in subframe #3 in downlink. In subframe #4, transmission is performed from relay station 20 to mobile station 30 again.
As described above, dividing communication into the backhaul communication and access link communication of relay station 20 in the time domain enables dividing the time into transmission time and reception time for relay station 20. Accordingly, relay station 20 can relay signals without being affected by coupling wave between a transmission antenna and a reception antenna.
MBSFN subframes are configured for the access links in downlink. “MBSFN subframes” are subframes defined for transmitting multimedia broadcast multicast service (MBMS) data. LTE terminals are configured not to use reference signals in MBSFN subframes.
In this respect, there has been proposed a technique that configures access link subframes that overlap a backhaul link subframe used by a relay station to communicate with a base station, for MBSFN subframes in LTE-A. This proposal allows LTE terminals to avoid erroneously detecting reference signals.
FIG. 2 illustrates an example of control signals and a data assignment state for each of base station 10, relay station 20, and mobile station 30 when subframes of the LTE system are used. As illustrated in FIG. 2, downlink control signals transmitted or received in each station are mapped in a control signal region in the top part of a subframe (hereinafter, referred to as “Physical Downlink Control Channel (PDCCH) region”). More specifically, both of base station 10 and relay station 20 transmit control signals in the PDCCH region in the top part of the subframe. With reference to relay station 20, downlink control signals (PDCCH) have to be transmitted to mobile station 30 even in an MBSFN subframe. Accordingly, relay station 20 transmits downlink control signals to mobile station 30 and then switches the processing to reception processing to prepare for receiving signals transmitted from base station 10. Base station 10, however, transmits downlink control signals intended for relay station 20 at the time relay station 20 transmits downlink control signals to mobile station 30. For this reason, relay station 20 cannot receive the downlink control signals transmitted from base station 10. In order to avoid such inconvenience, studies have been carried out on providing a region in which downlink control signals for relay stations are mapped (i.e., relay PDCCH (R-PDCCH) region) in a data region as illustrated in FIG. 2 in LTE-A.
(Control Signals)
In LTE, a base station transmits control signals to a mobile station using a downlink control channel such as PDCCH, for example. PDCCH includes DL grant indicating DL data (i.e., Physical Downlink Shared Channel (PDSCH)) assignment and UL grant indicating UL data (i.e., Physical Uplink Shared Channel (PUSCH)) assignment.
In LTE-A, studies have been carried out on including DL grant and UL grant in R-PDCCH. In addition, studies have been carried out on mapping the DL grant in the first slot and the UL grant in the second slot for R-PDCCH (see, Non-Patent Literature (hereinafter, referred to as “NPL”) 1). Mapping the DL grant only in the first slot reduces a delay in decoding the DL grant and allows relay stations to prepare for ACK/NACK transmission for DL data (e.g., transmission performed in the fourth subframe following the reception of DL grant in FDD).
In addition, studies have been carried out on allocating, for each relay station, a different physical layer resource block (i.e., physical resource block (PRB)) on which an R-PDCCH region is provided as illustrated in FIG. 3. In FIG. 3, the vertical axis indicates frequency and the horizontal axis indicates time. In FIG. 3, the R-PDCCH for relay station RN 1 is mapped on PRB #0 and the R-PDCCH for relay station RN 2 is mapped on PRBs #6 and 7 in the same subframe, for example. Each relay station finds the downlink control signals intended for the relay station by performing blind-decoding on downlink control signals transmitted using an R-PDCCH region from a base station within a resource region indicated using higher layer signaling from the base station (i.e., R-PDCCH search space).
(DM-RS Arrangement)
In LTE-A, studies have been carried out on introduction of demodulation reference signals (DM-RS) mainly for the purpose of directing a different beam for each mobile station and relay station. DM-RS is expressed by a combination of a plurality of ports defined as mutually orthogonal resources (e.g., ports 7, 8, 9, and 10) and scrambling IDs (SC-ID: SC-IDs 0 and 1) defined by randomization using non-orthogonal but different sequences. When DM-RS is used for channel estimation, a base station can use an optional beam by applying the same beam (i.e., same precoding) to DM-RS and control signals and data signals that pertain to the DM-RS.
FIGS. 4A and 4B are provided for describing mapping of DM-RS used for channel estimation. In FIGS. 4A and B, the vertical axis indicates frequency and the horizontal axis indicates time. Normally, DM-RS is mapped at the last two symbols of each slot (first slot and second slot) as illustrated in FIG. 4A. As illustrated in FIG. 4A, DM-RS (referred to as DM-RS ports 7 and 8 in FIGS. 4A and B) is mapped at OFDM symbols #5 and #6 as well as OFDM symbols #11 and #12 in a normal subframe. In addition, as illustrated in FIG. 4B, studies have been carried out on signal arrangement without using the last symbol of the second slot (i.e., OFDM symbol #12) in the abovementioned TD relay for a case where UL data assignment is present in the immediately following subframe and the restrictions on signal transmission timing from a relay station to a base station are stringent including a case where the distance between the relay station and base station is long, for example. In FIG. 4B, DM-RS is mapped only in first slot without being mapped in the second slot for signal arrangement without using OFDM symbol #12.
In addition, since relay stations perform R-PDCCH blind-decoding as described above, the DM-RS used on R-PDCCH is fixed to port 7 and SC-ID=0, for example. Thus, each relay station can omit blind-decoding for ports other than port 7 and SC-IDs other than SC-ID=0, thereby reducing the number of blind-decoding attempts. In this manner, a simplification of the processing is achieved.
On the other hand, regarding PDSCH, each base station can explicitly report the port used for PDSCH using DL grant. Thus, each base station can perform a Single User Multiple-Input Multiple-Output (SU-MIMO) operation to transmit PDSCH intended for the same relay station using a plurality of beams, or a Multi User-MIMO (MU-MIMO) operation to transmit PDSCHs intended for a plurality of relay stations using different beams, respectively.
Moreover, since no other signals are transmitted in the region of the R-PDCCH region in which DM-RS is transmitted (hereinafter, referred to as “DL grant region”), rank-1 transmission is used for DL grant regardless of the number of transmission beams for PDSCH (hereinafter, referred to as “rank”). For this reason, the accuracy in detecting DL grants in each base station can be improved by application of power boost that allocates all the power assigned to the resource on which DL grant is mapped to DM-RS and DL grant.
(PRB Bundling)
In addition, studies have been carried out on PRB bundling as a technique for improving the accuracy of channel estimation. PRB bundling is a technique that uses the same precoding for a plurality of PRBs adjacent to each other when a different beam is directed to each relay station and mobile station using DM-RS, thereby improving the accuracy of channel estimation (see, section 7.1.6.5 of NPL 2, for example). In PRB bundling, the receiving side performs averaging or interpolation of channel estimation values calculated using DM-RS mapped on RBs, in units of sets of adjacent PRBs to which the same precoding is applied (hereinafter, referred to as “Precoding Resource Block Groups (PRGs)”), for example.
(PRG Size)
The number of adjacent PRBs to which the same precoding is applied is referred to as “Precoding Resource Block Group (PRG) size.” The value configured for PRG size varies depending on the number of RBs included in the system bandwidth (hereinafter, referred to as “RBs”). FIG. 5 illustrates a correspondence between the number of RBs in the system band, the PRG size and RBG size (Resource Block Group size). The term “RBG” as used herein refers to a unit obtained by bundling one or more RBs. As illustrated in FIG. 5, the PRG size and RBG size are determined according to the number of RBs in the system band.
As the PRG size increases, the DM-RS to which the same precoding is applied increases. As a result, the reception performance with respect to DM-RS (e.g., Signal to Noise Ratio (SNR)) can be increased. Meanwhile, as the PRG size increases, the influence of frequency-selective fading is more likely to become uneven on each DM-RS, which in turn increases the possibility that the optimum beam is not configured even when the DM-RS is used. For this reason, even when the number of RBs in the system band is large, the PRG size is configured to be 2 or 3, considering a tradeoff between the abovementioned “effect of increasing the reception performance with respect to reference signals” and “influence of frequency-selective fading” as illustrated in FIG. 5.
(R-PDCCH Aggregation Size)
In addition, in order to adjust the coding rates of DL and UL grants according to the channel quality as in the case of PDCCH in LTE Release 8, studies have been carried out on preparing a plurality of aggregation sizes (may be referred to as “control channel element (CCE) aggregation size”). FIG. 6 is a conceptual diagram illustrating R-PDCCH aggregation sizes. In FIG. 6, each vertical axis indicates frequency and each horizontal axis indicates time. As illustrated in FIG. 6, as the R-PDCCH aggregation size is reduced from 8, 4, 2 to 1, the coding rate is increased. In other words, a smaller aggregation size is suitable when the channel quality between a base station and a relay station is good.
For example, a base station estimates the channel quality between the base station and a relay station and determines the R-PDCCH aggregation size and transmits signals in accordance with the determined aggregation size to the relay station. Meanwhile, the relay station is not previously informed of the aggregation size, which is changed for each subframe. In this respect, the relay station performs blind-decoding for each of the plurality of aggregation sizes (e.g., aggregation sizes 1, 2, 4, and 8 in FIG. 6).